Defrosting system for heat exchange devices



Sept. 22, 1953 i c. PELLEGRINI DEFROSTING SYSTEM FOR HEAT EXCHANGE DEVICES Filed Feb. 11, 1948 WYEIWUR Lou/3 Pd/egrini Patented Sept. 22, 1953 UNITED STATES PATENT OFFICE DEFROSTING SYSTEM FOR HEAT EXCHANGE DEVICES Louis C. Pellegrini, St. Louis, Mo.

Application February 11, 1948, Serial N 0. 7,584

6 Claims.

quently where the heat exchange devices are operated at temperatures below the freezing point ofwater; and in many instances the deposits of frost on the heat exchange devices can become quite thick. Such deposits will, unless periodically removed, reduce the heat transfer rate of the heat exchange device and thus reduce the efiiciency of that device. Accordingly it has be-- come customary, in the operation of heat exchange devices that tend to become frosted, to periodically raise the temperature of those heat exchange devices to levels at which the frost can melt and run oif of those devices. This operation shouldbe performed quickly so the temperature of the air being cooled is not permitted to rise unduly.

. Numerous methods and apparatus have been proposed and used for defrosting heat exchange devices; and while each of those methods and apparatus contemplates the cessation of normal flow of refrigerant through the heat exchange device, the raising of the temperature of that device to a level at which the frost melts, and the resumption of normal flow of refrigerant, the methods and apparatus are individually different. One such method positions electric heating elements in heat-transferring relation with the heat exchange devices; and those electric heating elements provide radiant and conducted heat that raises the temperature of the heat exchange device to a level at which the frost will melt and run off of that device. The electric heating elemerits are quite workable, and they quickly defrost the heat exchange devices; but the use of such heating elements necessitates the provision of a large number of electrical connections. Those electrical connections will be subject to pcriodic and sizable changes in temperature, and they will also be subject to contact with water in liquid and solid form. These two factors impose serious operational limitations on electrical heating elements placed in heat-transferring relation with heat exchange devices. In addition, in order to transfer their heat to the heat exchange devices as efficiently as possible, the electric heating elements should be given direct and intimate metal-to-metal contact with the heat exchange devices; but that contact can make it difficult to replace aged heating elements, particularly when corrosion products form between the electrical heating elements and the heat exchange devices.

Another method, that has been used in defrosting heat exchange devices, contemplates shutting off normal refrigerant flow through the pipes of the heat exchange devices, passing hot refrigerant through those pipes until the temperature of the heat exchange devices rises to a level at which the frost melts, and then resuming normal refrigerant flow through those pipes. This method is workable; but it must be performed with great care by very skilled operators, or it can cause considerable damage to.

the refrigeration system. For example, unless great care is used, particles of liquid refrigerant can enter the inlet of the compressor and cause serious damage to the compressor.

Another method of defrosting heat exchange devices contemplates spraying brine or water over the heat exchange devices. The use of brine necessitates frequent reductions in the volume of the brine, because the melted frost, together with condensed water vapor entrained by the brine, unduly increases the volume of the brine. This increase not only complicates the problem of handling the brine but it dilutes the brine. The use of tap water is objectionable because of the quantities of water required and because a slow draining nozzle or outlet could freeze and burst. In addition, it is oftentimes difiicult, in view of local conditions, to obtain an adequate supply of water for the defrosting operation. This is particularly true on shipboard where the water of harbors tends to become fouled with sewage and refuse of various types.

For these various reasons, prior defrosting methods and apparatus are objectionable. The present invention obviates these various objections by providing a defrosting apparatus which includes a heating system that has heat-transferring tubes in contact with the heat exchange device and has arfiuid that passes to those tubes to heat the heat exchange device. The tubes of the defrosting apparatus of the present invention provide a fluid path that is separate and distinct from the refrigerant path'of the heat exchange device; and thus no change need be made in the direction of flow of refrigerant through the heat exchange device. The defrosting apparatus of the present invention obviates the need of water and brine for defrosting purposes, and thus it practically eliminates all dangers inherent in slow draining nozzles. In addition it avoids the problem of periodically reducing the volume, while maintaining the salt concentration, of brine. For these reasons, the defrosting apparatus of the present invention is quite efficient and useful. It is therefore an object of the present invention to provide a heating system that has tubes in contact with the heat exchange device and has a fluid that passes tothose tubes to defrost that device.

The defrosting apparatus of the presentinvention preferably constitutes a closed heating system. Such a system avoids loss of the heating fluid and thus avoids the need of supervising and regulating the amount of fluid provided for the system. With the closed heating system of the present invention, the initial charge of fluid lasts for the lifetime of that system. It istherefore an object of the present invention to provide a defrosting apparatus that constitutes a. closed heating system.

The defrosting apparatus of the present invention preferably employs a volatilizable fluid. Where this is done, the fluid can volatilize and pass to the tubes in contact with the heat exchange device. Those. tubes can then transfer the sensible and latent heat of the vaporizable fluid to the frost on the heat exchange device, thus providing quick defrosting of that device. If the vaporized fluid is condensed as it defrosts the heat exchange device, it will flow back down to the source of heat where it can be reheated. Such a defrosting apparatus needs no pumps to move the fluid, needs no valves to control the direction of flow of the fluid, and needs no replenishment of the vaporizable fluid. It is therefore an object of the present invention to provide a defrosting apparatus that constitutes a closed heating system.

The defrosting apparatus of they present invention preferably has a heating chamber or boiler; and that chamber may be heated in different ways. Where the heating chamber is to be heated by electric heating elements, those elements should be completely. isolatedfrom contact with the water or, moisture adjacent the heat exchange device. Where this is done,,pr,ob lems due to corrosion of the heating elements are obviated. The present invention makes this possible; and it is therefore an object of the present invention to provide a defrosting apparatus wherein the heating elements are isolated from the water or moisture adjacent the heat exchange device.

In. operating the defrosting apparatus of. the present invention, it is preferable to employ a fluid that can be vaporized at pressures below atmospheric pressure. Where this is done, a leak or break in the heating system will cause air to be drawn into the system rather than permit vapors to issue out from the heating system. Moreover, by limiting the temperatures to which the fluid is heated. the heating system of the present invention can keep the fluid from vaporizing and issuing. out of the system; because a leak would permit the pressure in the system to rise to a level at which the. fluid would not vaporize within the permitted temperature. limits. Such an arrangement not only provides a very safe heating system but it also provides a system wherein leaks can be promptlydetected, because. the heat exchange device would not defrost although the heating chamber was warm. It is therefore an object of the present invention to provide a defrosting system for heat exchange devices wherein the pressure is below atmospheric pressure.

Other and further objects and advantages of the present invention should become apparent from an examination of the drawing and accompanying description.

In the drawing and accompanying description two preferred embodiments of the invention are shown and described but it is to be understood thatthe drawing and accompanying descriptions are. for the purposes of illustration only and do not limit the invention and that the invention will .bedefined by the appended claims.

In the drawing;

Fig. l is a partially broken away end View of a heat exchange device and defrosting apparatus therefor that are made in accordance with the. principles and teachings of the present in- YEI'ltlOI-L, V

Fig. 2. is a side elevational view of. the heat exchange device and defrosting apparatusshownin Fig.1, and

Fig. 3 is an end View of a modified form of heat exchange device and defrosting apparatus therefore that are made in accordance with the principles and teachings of the. present invention- Referring to the drawing in detail, the numeral Ill denotes a housing for a heat exchange device. That housing is preferably made ofstout mate! rial so it can support the heat exchange deviceand the defrosting apparatus therefor and that housing is shown as being made of insulating material. Thehousing is provided with suitable openings, not shown, that permit ingress and egress of the air or .gas. to be. treated by the heat.

exchange device within the housing. A blower or fan will customarily be used to move air through the. housing |0- where forced draft is desired; however, the heat exchange device can be of the type where convection currents move the air.

The numeraln denotes horizontally disposed pipes which are shown diagrammatically, and which cooperate with a number of. similarly shown return bends M to form. a heat exchange device. The pipes i2 are parallel to each other and are held fixed relative to each other; as by being positioned in spaced openings in heat transferring fins L5. The fins 1-6, which are par allel to each other and which are transverse. to the pipes I Lare'made to closely engage the pipes [2; thus the fins It can readily transmit heat to" and from the pipes Ill.

The pipes I12 and the return bends I4 constitute a closed system for refrigerant a refrigerant. line It being connected to one-end of that system and a refrigerant line 20 being connected" to the other end of that' system; The pipes t2 and return bends M can form a. dry or flooded or gas will. tend to. condense on the surface of pipes 112, return bends l4, and. fins Hi. If the.

temperature of pipes 12, return bends l4, and fins I6 is low enough, the condensed moisture will freeze and form a deposit of ice or frost on the heat exchange device. That deposit will reduce the heat transferring capacity of the heat exchange device and will offer increased resistance to the flow of air or gas over the heat exchange device. Unless the deposit of frost or ice is removed periodically, the efiiciency of the heat exchange device will be reduced. The present invention provides a method and apparatus for periodically removing that deposit; and one component part of that apparatus is a heating chamber or boiler 22 which is positioned below the heat exchange device. The upper surface of the heating chamber 22 is positioned immediately adjacentthe bottom of the fins is of the heat exchange device; and those fins may, if desired, rest on that surface. Thus the heat from the chamber 22 can pass to the fins l6 by convection and .conduction. The heating chamber 22 is shown as being of semi-cylindrical form; and such a form is desirable because it is readily fabricated and because it enables small quantities of liquid in the chamber to gravitate to the bottom of the chamber and surround the two longitudinally extending tubes 24 located at that point. However, it should be obvious that other forms of heating chambers could be used. The tubes 24, which are hollow and which have their right hand ends closed, extend through the length of the chamber 22; and the left hand ends of those tubes extend sufliciently far out of the left hand end of the heating chamber 22 to extend through the left hand wall of housing 18. The tubes 24 are suitably sealed into the left hand wall of housing I0, as by caulk or cement, so melted ice or frost from the interior of housing It! cannot escape through the openings for the tubes 24. The tubes 24 are also sealed to the heating chamber 22, as by soldering, brazing, welding, or the like, so the melted ice or frost from the heat exchange device is completely isolated from the interior of the tubes 24. This isolation makes it possible to position electrical heating elements 23 within the tubes 24 and yet keep those elements free of melted ice or frost. The heating elements can be inserted into the open left hand ends of tubes 24 and pressed inwardly until they are in register with the heating chamber 22. The leads from the heating elements will extend out from the left hand end of tubes 24 into terminal box 25; and those leads can readily be connected to a source of electricity.

When the heating elements are energized they will conduct and radiate heat into the tubes 24; and those tubes will transmit that heat to the liquid contents of the heating chamber 22. Those liquid contents will, when heated to the required temperature, vaporize and flow into a header 26 that extends transversely of the chamber 22. A valve 28 is positioned between the heating chamber 22 and the header 26; and that valve is usable, at the time the heating chamber 22 is initially charged with its liquid contents, to evacuate the chamber and the piping of the defrosting apparatus and then to permit insertion of a, predetermined amount of the liquid. The

header 26 communicates with a number of vertically disposed pipes 30 at the left hand end of the heat exchange device; and those pipes communicate with a number of horizontally disposed pipes 32. The horizontally disposed pipes 32 extend through, and are positioned by, openings in the fins I6 of the heat exchange device; and

the fins l6 intimately engage the pipes 32 and can absorb heat from those pipes. Because the fins 16 are also in intimate contact with the pipes [2, the fins I6 can transfer heat from pipes 32 to pipes l2.

The other ends of the horizontally disposed pipes 32 are connected to a header 36 by vertically disposed pipes 34; and the header 35 is directly connected to the chamber 22. The pipes 32 cooperate with pipes 30, headers 26 and 36, and chamber 22 to provide a closed heating system; and the liquid and vapor in that system can be completely isolated from the air or gas in housing l0 and from the refrigerant in pipes l2 and return bends Hi. When heat is applied to the chamber 22, the liquid will begin to vaporize; and that vapor will rise upwardly through the vertically disposed pipes 30 and 34 and pass into the horizontally disposed pipes 32. The pipes [2 will be cold, relative to the temperature of the chamber 22, and the vapor will transmit its latent and sensible heat to pipes 32. That heat will pass to fins I6; and those fins will conduct a good part of that heat to the pipes I2, becoming heated themselves in the process. Very quickly the temperature of the fins l5 and pipes l2 will be raised to a level at which the ice and frost will melt. The melted ice or frost will drop to the bottom of housing [0 and pass through drain 33 to sewage facilities, not shown. Those facilities should be large so the melted ice or frost will not freeze plug or burst those facilities.

In the process of heating pipes 32, fins It, and pipes l2, the vapor in the pipes 32 will condense, the resultant liquid gravitating to the left hand or right hand ends of the horizontally disposed tubes 32 and draining back down to the heating chamber 22. There it can be reheated and passed again to the pipes 32. If desired, it would be possible to incline the pipes 32 to the horizontal and thus facilitate draining of the condensed vapor; but draining of the condensed vapor is not a problem because only a film of liquid would remain in the pipes, and that liquid will usually have a freezing point below the normal range of temperatures experienced by the heat exchange device. Moreover, heat exchange devices are rarely set exactly level and the liquid will fiow to the lower ends of pipes 32.

It is desirable to maintain a reduced pressure in the defrosting apparatus at all times, and such a reduced pressure is made possible by initial evacuation of the apparatus, selection of a vaporizable liquid with a low vapor pressure, and insertion of a limited amount of the liquid in the apparatus. It can also be done by inserting the liquid in the apparatus,, heating the liquid until it vaporizes and expels most of the-air from the pipes and chamber, and then sealing the system. In both instances, there will be a partial vacuum in the pipes 32 when heat is applied to chamber 22; and that partial vacuum will permit prompt movement of vapor to pipes 32. The condensation that occurs when the vapor contacts the cold surfaces of pipes 32 restores the partial vacuum and permits further movement of vapor to those pipes. Consequently, even though the defrosting apparatus constitutes a closed system, vapor will fiow from the source of heat to the cold areas in the pipes 32. The condensed vapor will be returned to the chamber 22 by the action of gravity.

- The contents of the heating chamber 22 have been shown as being heated by electrical heating elements, but the present invention is not limited assaeorz 7 to electrical. heating elements. since the products of combustion from solid, liquid or gaseous'fuels could be used to heat the vaporizing chamber 22.. In addition, hot fluids such as water, steam, .or high pressure refrigerant could be used. to heat the chamber 22. With any of these possible methods of heating the vaporizing chamber 22, all that need be done is to extend the tubes 2:4.so

both ends project from the. housing 10. and then.

pass the hot medium through thosev tubes. High pressure refrigerant can be used to heat chamber 22 where a single compressor serves two or more: heat exchange devices or where two compressors, in proximity to each other, serve two or. more heat exchange devices. A portion of the high pressure refrigerant, intended for the heat exchange device which is providing a cooling effect, can 'be diverted to the heatingchamber for the heat exchange device to be defrosted. Because-the defrosting apparatus of the present invention can use so many different methods of heating, it can be adapted to use the cheapest and .mostreadily available heatin any locale.

Where electric heating elements are used toprovide heat for the heating chamber 22, a heat sensitive bulb 38 can be attached tonne of the pipes 30 that extends. upwardly from the header 2B. This bulb cooperates with a controlling mechanism 41, of usual form and design, to selective'ly determine the energization and deenergization of the electrical heating elements posi tioned in the tubes 24.. Thebulb 38 and the control 4| are preferablyset todeenergize those heating elements when the temperature of the pipes 30 reaches a predetermined level and to reener gize those heating elements when the temperature of those pipes falls below that level during a defrosting period. A master switch, not shown, which will preferably be a time switch, will be operated to determine the initiation and completion of the defrosting period. Where other methods of heating are used, the heat sensitive bulb 38 and the control M will be connected to regulate the introduction and combustion of the fuel or heated fluid in accordance with the tem perature of the pipes 39.

It is preferable, in defrosting theheat exchange device, to halt the flow of refrigerant through pipes 12, to close the openings in the housing l which normally permit ingress and egress of air or gas, and to discontinue operation of the fan or blower. Such operations keep the heat in pipes 32'and fins Hi: from raising the temperature of the mass of air or gas being cooled. Once those openings are closed. and the operation of the fan or blower discontinued, the heating elements are energized to heat the chamber 22 and cause vapor to form and melt the ice and frost on the coil. Once the ice and frost has been melted, and this can be assured by visual examination of the heat exchange. device, by energizing the heating elements for a period of time sufllcient to melt all of the ice and frost, or by using a thermally responsive device to continue the heating cycle until the temperature of. the entire heat exchange device reaches a level well abovethat of the freezing point of water, the heating elements are deenergized The melted ice and. frost will drain off of the pipes l2, fins i5 and pipes 32, and the device will be ready for further operation. Such operation can be commenced by re-starting the flow of refrigerant, by re-opening the openings in housing 1'0 and by re-starting the fan or blower.

In Fig. 3, a modified form of defrosting appa- 8' ratus is provided; and. that apparatus includes a heating chamber 38. This chamber differs from the heating chamber 22 of Figs. land 2 in that the chamber 38 is circular and is not positioned directly beneath the heat exchange device with which it is used. Instead, the chamber 38is displaced to one side of the heat exchange device. Such an arrangement is desirable where a large number of individual heat exchange devices are to be defrosted and it is not economical to place a heating chamber under each of those devices. Moreover, it is sometimes desirable to place the heating chamber outside of the housing for'the heat-exchange device and outside of the room or space beingrefrigerated.

The heating chamber 38 is provided withlongitudinally extending tubes 40 that can receive electrical heating elements or that can receive a. heated medium such as products of combustion, hot water, steam, high pressure refrigerant, or the like. A header 42 extendsiupwardly from one end of chamber 38, and that header is connected to av number of. horizontal tubes, not shown, by connecting tubes 4'4. A second header 4.3 isconnected to the said one end of the vapporizing chamber and that header is connected to horizontally disposed tubes, not shown, by connecting tubes 48. The horizontally disposed tubes fit into openings inufins l Bi of the heat exchange device and they are held firmly by those fins. The ends of those tubes, opposite headers 42 and 48 are connected together by vertically disposed tubes, not shown, that cooperate with. chamber 38, header 42 and 46, the horizontally disposedv pipes, not shown, and the connecting tubes 44 and 48 to form a. closed heating system. When the liquid contents of this system are heated, as by energizing heating elements inserted in the tubes 43 of chamber 38, vapor will pass into the horizontally disposed :tubes. That vapor will transfer its sensible latent heat to the horizontally disposed tubes, thus causing heating of those tubes, the fins I26, and the pipes of. the refrigeration. system. Any vapor thatcondenscs in the horizontally disposed tubes will drain back to the chamber 33 through headers 42 and 45. .A bleeder connection to provided in the header 42; and that connection is usable to evacuate the defrosting apparatus and to introduce the liquid to be heated in the chamber 38.

Various liquids could be used in the defrosting apparatus of. the present invention, but low melting point, non-corrosive, readily volatilized liquids are preferred. While certain materials which are liquid at ordinary temperatures, as for example mercury and some silicones, can be heated and circulated through the pipes 32 :of the defrosting apparatus, those materials must be moved by a pump. In addition those materials can only, within reasonable temperature ranges, transfer sensible heat to the pipes 32; while vaporizable liquids can transfer both semi-- ble and latent heat to those pipes. Some of. the

liquids which are usable in the defrosting ap paratus provided by the present invention are: the group of chlorinated hydrocarbons which are identified by the word Freon and a num ber designation, as for example Freon 11 and Freon 113, trichlorethylene, carbon tetrachloride, perchloroethylenc, methylchloride, methylene chloride, chloroform, ether, and low molecular weight alcohols, as for example methyl alcohol, ethyl alcohol, propyl alcohol butyl alcohol and amyl alcohol. One volatile liquid that is particularly useful is Freon 11 I (CC13F), which will, under atmospheric pressure, vaporize at temperatures of about seventy-four and seven tenths (74.7) degrees Fahrenheit, and it will freeze at minus one hundred sixty-eight (-468) degrees Fahrenheit. While it is entirely feasible to charge the defrosting apparatus at atmospheric pressure, it is preferred to charge that apparatus at pressures below atmospheric pressure. Where this is done, the vaporizing temperature is kept quite low, and the dangers inherent in leakage are minimized. For example. where Freon 11 is charged into the defrosting apparatus of the present invention at a pressure of fifteen (15) inches of mercury below a standard atmosphere, or at an absolute pressure of seven ('7) pounds per square inch, the Freon 11 will vaporize at temperatures of about forty (40) degrees Fahrenheit. The control 4| and the bulb 38 could be adjusted to deenergize the heating elements whenever the temperatures in the pipes rose above fifty or sixty (50-60) degrees Fahrenheit; and with such an arrangement, if the apparatus develops a leak, air will pass into that apparatus instead of the Freon 11 leaking out. Moreover, the infiux of air into the apparatus, that occurs when a leak appears in the apparatus, will raise the pressure in the apparatus to atmospheric pressure, thus halting vaporization of the Freon 11 since that liquid cannot vaporize at temperatures of fifty to sixty (50-60) degrees Fahrenheit under conditions of atmospheric pressure. This immediately provides the defrosting apparatus with an important safety factor. In addition, it will also provide immediate notification of the existence of a leak because the vaporizing chamber 22 will be warm but the frost or ice on the pipes I2, pipes 32 and fins I6 will remain untouched.

The thermally responsive bulb 38 provides prompt and accurate control of the energization and deenergization of the heating elements positioned within tubes 24 of heating chamber 22. However, a thermally responsive bulb is not necessary; and a pressure-responsive element can be substituted for it. Because of the direct relationship between the temperature and pressure of vaporizable liquids, a pressure responsive or a temperature responsive bulb can be used interchangeably. Any pressure-responsive device should be tightly sealed, as by a Sylphon bellows, to prevent leakage.

It will be noted that in the heat exchange device of Figs. 1 and 2, the horizontally disposed pipes 32 are spaced quite a distance above the bottom of the fins I6 of that device. This is due to the fact that the heating chamber 22 is directly under the heat exchange device, and heat from that chamber will be radiated and conducted directly upward to the fins l6 and pipes I2 of the heat exchange device. With the defrosting apparatus of Fig. 3, the heating chamber 38 is not positioned directly under the heat exchange device, and thus it is desirable to have some of the pipes of the defrosting apparatus located at a lower point in the fins 16 of the heat exchange device.

The defrosting apparatus of Fig. 3 preferably is a closed heating system; but if a source of steam is readily available it would be possible to connect that source directly to the headers 42 and 46 and to provide those headers with air-venting valves. In such instances, as for example on shipboard or in refrigeration systems operated by steam, steam could quickly defrost the heat exchange device. Such an arrangement would obviate the need of a separate source of heat. In such an arrangement, the heating chamber 22 could be obviated or could be used and heated by steam introduced into tubes 24. The latter is more desirable since the Freon 11 has such a low freezing point that it would not freeze in the horizontally disposed pipes, even if it was prevented from draining out of those pipes.

Whereas two preferred embodiments of the present invention have been shown and described in the drawing and accompanying description, it should be obvious to those skilled in the art that various changes can be made in the form of the invention without affecting the scope thereof.

What I claim is:

1. A defrosting system for a heat exchange device that comprises a tube in heat transferring relation with said heat exchange device, a normally cool heating chamber, a conduit extending between and connecting said tube and said chamber, and a heat exchange medium that can receive heat from said chamber and can pass through said conduit to said tube to transfer said heat to said tube, said heating chamber being positioned beneath and immediately adjacent said heat exchange device whereby heat radiated from said chamber can heat said heat exchange device and whereby said conduit can be short.

2. A defrosting system for a heat exchange device that comprises a tube in heat transferring relation with said heat exchange device, a heating chamber, a conduit extending between and connecting said tube and said chamber, and a vaporizable fluid in said chamber, said chamber, tube and conduit constituting a closed system, said chamber being dimensioned to and normally acting to confine all of said fluid whenever said fiuid is in the liquid state, said chamber and said fluid being proportioned so said fluid can be maintained under a reduced pressure as long as said system remains closed, a source of heat for said chamber, and a thermally responsive control adapted to regulate the application of heat to said chamber.

3. A defrosting system for a heat exchange device that comprises a tube in heat transferring relation with said heat exchange device, a heating chamber, a conduit extending between and connecting said tube and said chamber, and a vaporizable fiuid in said chamber, said chamber, tube and conduit constituting a closed system whereby said fluid can be maintained under a reduced pressure, a source of heat for said chamber, and a thermally responsive control adapted to regulate the application of heat to said chamber, said control being operable to maintain the temperature in said chamber below the level at which said fluid can vaporize at atmospheric pressure.

4. A defrosting system for a finned heat exchange device that comprises a heat transferring tube secured to and supported by said fins, a normally cool heatin chamber, and a conduit that extends between and connects said tube and said chamber, and a heat exchange medium in said chamber, said chamber being positioned below and immediately adjacent the fins of said heat exchange device whereby heat from said chamber passes directly as well as indirectly to said heat exchange device, said fins acting as fiues to conduct heat from said chamber through said heat exchange device.

5. In a heat exchange device that comprises a housing, a heat transferring surface within said housing, and a heating, chamber; within said housing to supply defrosting heat to; said heat transferring surface, the, improvement which comprises a tube thatrhas one end-thereof extending into said heating chamber and has the other end thereof extending outofisaidhousinE. and an electric heating element disposed within said tube, said heating element having, the terminals thereof adjacent said other end, of. said tube, whereby I said terminals are, isolated .from the condensed moisture within saidhousing.

6. A defrosting system vfora heat exchange device that comprisesa tube in .heat transferring relation withsaid heat exchange device, aheating chamber, a sourceof ,heat for said heating chamber, said source of heat normally be ng ,deenergized so that, said chamber isnormally cool, a conduit extending between .and connecting said tube and said chambena vapor'izable fluidthat is normally held in the liquidlstatejinsaid chamber but can respond to, heat from said sourceofheat to vaporize andrfill said tube andconduit, andia temperature-res pons'ive,v element responsive, to the temperature, of said vaporizable fluid'to deenergize said source of heat andlthereby limit the temperature of, lsaidvaporizable mud to rises:

up to, but not above its -boi1ing point, said chameber 'anditube and-conduit constituting a closed.-

system; the amount of said vaporizable fluid being proportioned-tothe volume of said closedssyse tern so that said vaporiza'blefluid maintained under e. reduced pressure throughout the entire defrosting cycle.

LOUIS C. PELLEGRINI.

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